1. Field of the Invention
This invention relates to a steering wheel assembly, a mounting structure thereof and a boss structure thereof.
A steering wheel assembly comprises an annular ring, a pad disposed in the center of the ring and spokes extending from the ring to the pad. A ring core bar is embedded in the ring, while spoke core bars are embedded in the spokes. In this case, the ring and spoke core bars are covered with a sheath made of polyurethane foam, for example. A Distal ends of the spoke core bars are connected to a boss plate (boss) below the pad. The boss has a through hole with a serration on an inner periphery thereof so that the boss is fitted on an end of a steering shaft formed with the same serration and mounted thereon by tightening a nut.
In the steering wheel with such construction, the spoke core bars and hence the boss integrated with a steering wheel main body are mounted on the steering shaft by tightening the nut, and Thereafter an air bag device and the like are attached and the pad is fitted to cover them.
On the other hand, recently there has been proposed a steering wheel of the type that a sheath and a pad are formed integrally to eliminate a border line between the sheath and pad (Japanese Utility Model Unexamined Publication No. 2-133955, for example). According to such technology, since the sheath and the pad can be integrated, it is possible to improve the design.
However, according to the above technology, it is very difficult to tighten a nut after fitting the steering wheel main body on the end of the steering shaft. This is because the top of the steering wheel main body is not open since the pad is integral with the sheath.
In contrast, it can be thought to adopt a yoke type boss so as to tighten the steering wheel from the side thereof with a bolt. Namely, the yoke type boss includes a cylindrical portion having a serration corresponding to the serration of the steering shaft formed on an inner periphery thereof and a yoke portion formed integrally with the cylindrical portion. The yoke portion is of generally C-shape in plan and is formed with through holes substantially at both ends thereof. At least one of the through holes is formed with an internal thread. Meanwhile, the steering shaft is formed with a constricted portion in the position corresponding to the through holes.
By screwing the bolt into the through holes, the serrations are caused to mesh with each other and the yoke portion is tightened. This contributes to restraining the steering wheel from moving in the direction of rotation. At the same time, a shaft of the bolt is fitted in the constricted portion. This contributes to restraining the steering wheel from moving in the vertical direction as well. As a consequence, according to such technology, even in the steering wheel in which the pad is integral with the sheath, the mounting operation can be performed relatively easily.
However, in the above described conventional technology, the shaft of the bolt by which the steering wheel and the steering shaft are connected, is designed to be fitted directly in the constricted portion of the steering shaft. Therefore, when external stress is applied to the steering wheel, stress strain between the steering wheel and the steering shaft is transmitted directly to the bolt. As a result, there has been an apprehension that after assembling the steering wheel, the tightened bolt might be loosened or the threaded portion (internal or external thread) might be damaged.
In the field of steering wheels, it is common to make spoke core bars and the like made of die-cast aluminum so as to make the steering wheel lighter as a whole. In this case, it is necessary to connect aluminum spoke core bars to an iron boss. In the case of the usual boss (formed in a generally cylindrical shape), a preformed boss is set in a mold for die casting as an insert and, in this condition, molten aluminum is poured into the mold. Then, the core bars can be formed in such a manner that the periphery (extreme edge) of the boss is connected to the spoke core bars, that is, the extreme edge of the boss is embedded in aluminum to form the core bars.
However, such technology could not be applied as it was in cases where the above yoke type boss was formed in a generally C-shaped in plan so as to allow molten aluminum to go around through an opening of the yoke portion.
The above conventional technology, however, needed to have such a configuration in that it prevents molten aluminum from going around to the inside of the boss when casting the spoke core bars. For this reason, the shape itself was limited, resulting in low degree of freedom in its shape.
Particularly, in cases where the yoke type boss was adopted (which has a yoke portion for bolt tightening formed on the cylindrical portion to be generally C-shaped in plan), the above die casting technology could not be applied as it was. This is because the yoke portion of the boss is generally C-shaped in plan, so that molten aluminum goes around through the opening of the yoke portion at the time of die casting.
On the other hand, Japanese Patent Unexamined Publication No. 60-60065 discloses a technology in which spoke core bars are connected to each other by means of a connecting portion with a through hole in which the boss is to be fitted. Fixing of the boss is performed by caulking a peripheral edge of the through hole of the connecting portion. According to this technology, the boss can be fixed to the connecting portion after connecting the spoke core bars to each other, and therefore the degree of freedom in shape of the boss itself can be further increased.
However, in this technology, since the peripheral edge of the through hole of the connecting portion is simply caulked from the outside, there is the risk of easy rotation of the boss, resulting in difficulty in ensuring sufficient joining strength between the boss and the core bars. This gives rise to a problem with respect to stability of mounting condition.